1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor, as well as an image forming apparatus and a process cartridge using the photoreceptor, which realize high-speed printing, high-quality image production and high durability simultaneously.
2. Discussion of the Background
Recently, data processing systems using electrophotographic methods have made remarkable progress. In particular, laser printers and digital copiers which record data with light by changing the data into digital signals have made remarkable improvements in their printing qualities and reliabilities. Further, technologies used in these printers and copiers have been applied with high-speed printing technologies to laser printers and digital copiers capable of printing full-color images.
Recently, in accordance with high-speed printing of an electrophotographic apparatus, high sensitivity and response of an electrophotographic photoreceptor have been required, and further in accordance with downsizing of the apparatus, a smaller diameter or size and higher durability of the photoreceptor are required. To meet such requirements, Japanese Patent Publication No. 2838891 discloses a photoreceptor having both higher durability and sensitivity, wherein a specific charge generation layer (hereinafter referred to as a CGL) thereof has an electric field dependence of sufficiently a small quantum efficiency such that a gradient b is not greater than 0.5 in the following formula:η=a×Eb  (I) wherein η is a quantum efficiency of the photoreceptor, a is a constant, and E is an electric intensity, and wherein a charge transport layer (hereinafter referred to as a CTL) thereof has a thickness of from 25 to 60 μm.
Typically, a low-molecular-weight charge transport material used in a CTL is dispersed and mixed with an inactive polymer because the charge transport material is not capable of forming a film alone. A CTL formed of such low-molecular-weight charge transport materials and inactive polymers typically has a low hardness. Therefore, a coated film of the CTL is largely abraded due to repeated use in the Carlson process, resulting in deterioration of charge potential and photosensitivity of the resultant photoreceptor, background fouling due to a damage of a surface thereof and deterioration of the resultant image density and quality. Accordingly, it is preferable to increase thickness of the CTL to provide higher durability against the abrasion due to the repeated use.
In addition, typically in a multilayer electrophotographic photoreceptor, since a surface potential and a surface charge thereof are on a straight line passing through an original point, a superficial capacitance of the photoreceptor C=Q/V (C: a capacitance, Q: charge and V: surface potential) and C=εε0/d (ε: specific inductive capacity, ε0: vacuum inductive capacity and d: thickness of the photosensitive layer) can be obtained using a planographic condenser model. From these formulae, for a given surface potential it is found that a photoreceptor having a thin film has a large surface charge and a photoreceptor having a thick film has a small surface charge. When light having the same energy is irradiated to each of the photoreceptors and the same charge amount is generated, the surface charge of the thin film is not sufficiently extinguished and the irradiated part potential slightly decreases and the surface charge of the thick film is sufficiently extinguished and the irradiated part potential largely decreases. Namely, the thin film has less sensitivity than the thick film. Therefore, it can be said that the above-mentioned electrophotographic photoreceptor having a thick CTL has a higher sensitivity.
In the recent electrophotographic process, a so-called digital electrophotographic method writing an electrostatic latent image on a photoreceptor using a LD or LED is prevalent for high quality images, high-speed printing and expansible digital networks. As mentioned above, popularization of printers, copiers and full-color printers using such methods further requires higher image quality, higher speed printing and higher durability.
However, the above-mentioned electrophotographic photoreceptor having a thick photosensitive layer is disadvantageous for higher image quality because a charge injected in the CTL does not smoothly reach a surface of the photoreceptor to neutralize the surface charge, which causes blurred images. Further, the thicker the CTL, the influence significantly causes deterioration of the resultant image quality.
As a method of forming a photoreceptor having both high durability and image quality, it is attempted to form a protection layer to improve surface abrasion resistance of the photoreceptor and a thin CTL. However, the thin CTL causes deterioration of sensitivity as mentioned above.
In order to solve this problem, it can be considered that a charge potential is decreased. Namely, in the formula C=Q/V, even when the superficial capacitance becomes larger due to a thinner film, the lower surface potential V does not make the surface charge Q larger and prevents the deterioration of sensitivity. However, when the surface potential is decreased, a surface potential difference between an irradiated part and a non-irradiated part becomes small and it becomes difficult to form a sharp electrostatic latent image. Further, when a photoreceptor is electrostatically fatigued due to repeated use, chargeability of the photoreceptor deteriorates and residual potential thereof increases. Therefore, the surface potential difference between an irradiated part and a non-irradiated part becomes smaller, resulting in abnormal images such as background fouling and deterioration of image density.
As mentioned above, it has been quite difficult for a conventional image forming apparatus to realize high-speed printing, high-quality image production and high durability simultaneously.
Therefore, a need exists for an image forming apparatus realizing high-speed printing, high-quality image production and high durability simultaneously.